2 Answers
2

An optional letter indicating the intended vehicle class.
Your tyre doesn't have one (or you omitted it) but it should be a P for passenger car.
Possible values are:

P for Passenger Car

LT for Light Truck

ST for Special Trailer

T for Temporary

Digits before the slash indicate the tyre width in millimetres.
Your tyre has a width of 195 millimetres, which is a little over 7½ inches.

Digits after the slash indicate the height of the sidewall as a percentage of the width.
Your tyre has a sidewall height of 65% × 195 millimetres which equals 126.75 millimetres or about 5 inches.

The letter after that indicates the type of tyre.
Your tyre is an R for radial.
Possible values are:

B for Bias Belt

D for Diagonal

R for Radial

If omitted, it's a cross-ply

The digits directly after that letter indicate the diameter of the rim in inches.
Your tyre is mounted on a 15 inch rim.

The digits after the space are the load index.
Your tyre is rated for a load up to 615 kilogrammes (1,356 lb), which is per tyre. Assuming four tyres, that comes to a maximum total weight of 2460 kilogrammes of your car plus everything in and on it.
Possible values range from 60 for 250 kg / 550 lb to 125 for 1,650 kg / 3640 lb.

The letter after that is the speed rating.
Your tyre is rated V for speeds up to 240 km/h (149 mph).
Possible values are:

This is a very easy to understand breakdown of the previously confusing notation on the side of my tires. Thank you for the detailed clarification - exactly what I was looking for.
– Max GoodridgeJan 7 '16 at 12:37

Good job with the edit. I can now vote to close my other question on the same subject :)
– ZaidJan 8 '16 at 9:46

It means the surface is 195mm wide, the sidewall height is 65% of the width (195x0.65=127mm) and they fit a 15 inch wheel, 195-65-15. These give all the important dimensions of the tyres and shouldn't be changed. Doing so will give you a bigger or smaller tyre which could confuse your car's computers, causing ESP and ABS faults as well as affecting the accuracy of your speedometer.

91 is a load rating, you can look this up on a chart but 91 equates to a rating of 615 kilos. This is the maximum designed load to be carried by the tyre.
V is a speed rating, again you can look this up but V equates to 149MPH. This means the tyre is designed to cope with speed of up to 149MPH.

The load rating and speed rating are less critical if you stick to speed limits and bear in mind the weight of your car and its contents.

However bear in mind that fitting tyres with a different speed rating or load rating could invalidate your insurance, unless you declare it.
– Nick C♦Jan 7 '16 at 9:15

Thanks for that info, I'm only 18 so that is an important consideration for me. I guess I should stick with the 195-65-15 dimensions but wouldn't a tire like 205-60-15 give me more grip always with more surface area in contact with the road?
– Max GoodridgeJan 7 '16 at 10:18

1

I would concentrate on buying good quality tyres, rather than marginally wider ones. Wider tyres of the same type might very slightly increase grip, but fitting the wrong size could have an adverse effect. The manufacturer's engineers designed the suspension to work with a tyre that size, a different size may not work as well and could even reduce grip. A 205-60-15 has a slightly smaller rolling radius than a 195-65-15, which means your speedometer will over-read. Also, if you don't change all your tyres to the new size at once your ABS/ESP/TC systems may go into fault.
– SamJan 7 '16 at 10:41

1

The difference in circumference is marginal: 190.5 vs. 192.9mm makes 2.4mm (0.1") less. Speed will be about 1% lower than displayed, and I can't imagine that this could cause trouble with ESP etc. And yes, I agree: 10mm (0.4") wider tires won't give more grip.
– sweberJan 7 '16 at 10:49

I agree it probably won't make a difference in this case as the difference is within the wear limits of either tyre but I'd like to make the principle clear that (at least on modern cars) having odd sizes across axles can cause problems.
– SamJan 7 '16 at 10:59